1. Field of the Invention
The present invention relates to a method and to an apparatus for measuring and controlling the blow-up of a thermoplastic foam sheet, and more particularly, pursuant to controlling the oven operation through monitoring the blow-up or blown gauge of a material which being heated in a thermoformer oven as the material is conducted into a thermoformer or molding station, in order to be able to regulate the thickness or blow-up of the plastic material prior to its conveyance into the molding or forming station.
In recent years, molded articles which are constituted of a thermoplastic foam material have gained wide commercial and consumer acceptance due to their economics in manufacture, aesthetic appeal and excellent physical properties. Among such articles are egg cartons, meat and food trays, cups and even larger items, such as boat hulls, refrigerator liners or the like, which are readily molded from a thermoformable plastic material. For instance, it is possible to mass-produce thermoformed articles in an extremely economical manner by heating a continuous sheet of a thermoplastic foam material, for example, foamable polystyrene or similar materials, to a predetermined temperature in a thermoformer oven so that the material is in a thermoplastic state, and from there to convey the heated sheet into a molding station or forming press, in which the thermoformed article configurations are molded into the sheet of the plastic foam material.
In essence, the heating to the molding temperature of the thermoplastic foam material in the thermoformer oven not only plasticizes the material but concurrently therewith expands the foam material so that the thickness of the material is increased up to approximately 1.5 to 2.0 times the original thickness during its conveyance through the oven. Although this phenomenon, known as "sheet blow-up" is three dimensional in nature, orientation stresses purposely built into the material during extrusion thereof, opposed by the clamping action of the sheet gripping and conveying mechanism in the thermoformer oven, and the continuity of the sheet in the machine direction, result in minimal changes in the sheet length and width during heating of the latter in the thermoformer oven. The increase in the sheet thickness during heating is desirable since it reduces the density of the foam material and allows for the subsequent filling of wider mold gaps at various portions of specific foam plastic articles.
The mold gap in the production tooling, in effect, the void in the mold which is occupied by the product, may vary at different locations throughout the total mold cross-section for various reasons relating to desired final product shape, strength, functional needs, or aesthetics. Thickness of the expanded foam prior to entry into the mold is important since it is desirable to fill all portions of the mold gap throughout the various cross-sections of the mold cavity, and this must be accomplished while counteracting the thinning of the heated material as various portions are stretched and formed into the desired contours of the finished article.
In order to be able to provide molded products in the molding station which evidence a high degree of definition in its conformance with the configurations of the mold, and which provide a consistency and appearance essential to their saleability and utility, it is necessary to heat the plastic material to a predetermined temperature in the thermoformer oven and to maintain the material at that temperature to thereby achieve the desired extent of sheet "blow-up" or expansion. Variations in the temperature over the surface of the heated sheet of plastic material as it is conducted from the thermoformer oven into the forming press will adversely affect the uniformity in the foam material thickness and, as a consequence, the consistency and quality of the thermoformed articles.
Generally, the thermoformable foam plastic sheet is heated by radiant energy heating during its conveyance through the thermoformer oven through the intermediary of a series of heater elements which are usually arranged above and below the path of travel of the material. Such heater elements, which direct the radiant heat against the surfaces of the sheet of plastic material, may be constituted of suitable parabolic heaters, bar heaters, infrared heaters, or ceramic blocks having heating coils therein, which normally extend transverse of the direction of travel of the plastic material sheet through the thermoformer oven, and with the heater elements being energized from an external power source. Additional heater elements may be provided along the edges of the oven to compensate for heat losses to gripping devices which engage the edges of the sheet for transporting the latter through the thermoformer oven. Upon leaving the thermoformer oven, the thus heated thermoplastic sheet is introduced into a thermoforming press and interposed between the surfaces of a pair of mold members which cooperate to force or mold the heated thermoplastic material into the shape of the thermoformed articles defined by the contours of the mold surfaces.
2. Discussion of the Prior Art
Currently, temperature conditions within the thermoformer oven, which are assumed to be indicative of the temperatures to which the plastic material has been heated and the blow-up thereof, are usually monitored by temperature sensors or thermocouples which measure the temperature of one of the heater elements in each heat zone within the thermoformer oven; of particular importance being the temperature of the material at the discharge end of the thermoformer oven. Any differentials in the monitored or sensed temperatures which deviate from desired temperature levels are read by an operator who will then manually regulate the energy input to various of the heater elements in order to either raise or lower the temperature in those zones of the oven so as to provide and maintain the optionally desired thermoplastic material temperatures and resultant blow-up.
Although monitoring or sensing the temperature conditions within the thermoformer oven and controlling the amount of radiant heat generated by the heater elements will, in theory, be an indication of the plastic material temperatures and, as a result, the theoretical "blow-up" or increase in thickness of the material, in actuality considerable deviations have been encountered in the thickness of the foamed material after heating in comparison with the presumed final ideal thickness of the material. These differences between the actual blow-up thickness and the intended or contemplated thickness of the foam plastic material may be occasioned, among other factors, by the cycling on and off of the heaters during normal operation, low or high spot temperatures at various locations within the oven, or varying conditions within the oven, all of which tend to adversely affect the uniformity or extent of heating of the plastic material and, as a result, the extent of blow-up and consistency of any thermoformed articles produced from the non-uniformly heated material, possibly resulting in voids, uneven articles and poor molded definition of the product.
While the actual thickness of the expanded foam material is of particular importance relative to the quality and shape of the resultant thermoformed article, previously no means had been available to routinely monitor this process factor prior to the molding cycle. Rather, it had been the practice of the equipment operator to occasionally observe formed product exiting from the production mold either visually or by gauge measurement, and react to heat related product deficiencies via manual adjustments to one or more of the various oven heat zones, without knowing the actual sheet thickness as the sheet enters the forming mold.
More recently, there has been developed an apparatus possessing sensing devices of either a contact or non-contact nature which will measure and display the thickness of the foam material just prior to entry into the product molding section of the thermoforming installation. Additional thickness sensing devices in the sheet heating environment can also be incorporated to monitor the change in thickness of the thermoplastic foam material as it moves along the length of the oven and the heat is absorbed and/or the use of the measured thickness as a control element for the operation of the thermoformer oven. Such an apparatus and a method for its utilization is disclosed in Holden U.S. Pat. No. 4,438,054, commonly assigned to the assignee of this application.
The Holden U.S. patent specifically provides for the sensors within the thermoformer oven which measure the "blow-up" or thickness of the heat expanded foam material at one or more locations within the oven. The sensors, which may be in the nature of structural components either contacting the surface of the foam material, or non-contacting sensors, such as infrared detectors, are connected to oven controllers which afford visual readouts of the measured thickness to an operator. The oven controllers are in turn, connected to the heater elements of the oven so that, when the thickness or "blow-up" which have been sensed deviate from a desired thickness for the foam plastic material which is being heated during its passage through the thermoformer oven, the operator may vary the energy or power input to the heater elements in that zone or zones of the oven so as to either increase or decrease the heat radiating against the sheet of plastic material. This will permit for the exercising of a control over the oven operation so as to achieve a more precise and uniformly distributed heating and heat expansion or blow-up of the plastic material prior to the discharge thereof to the thermoforming press.
Other types of film gauge or blow-up monitoring or sensing and controlling arrangements are disclosed in U.S. Pat. Nos. Sagane, et al. 3,711,584; Eisenberg 3,801,244; Kiyono, et al. 4,213,925; and Moon 4,244,897. However, none of these publications measure the thickness or gauge of the material as the latter enters the forming station from the thermoformer oven.